The present invention relates to a process and apparatus for recovering hydrocarbons from air-hydrocarbon vapor mixtures and, more specifically, relates to a process and apparatus for efficiently collecting and recovering gasoline vapors which are normally associated with loading facilities where gasoline is transferred at which time such vapors are generated and must be disposed of to meet the current environment regulations.
As the environmental regulations have come into being in the last decade, a market has developed for equipment which would be used at gaoline loading terminals to prevent the emission of gasoline vapors to the atmosphere. While there are a number of different approaches used commercially to collect and recover these vapors, one of the preferred approaches involves the use of activated carbon adsorber beds which are used alternatively to adsorb the hydrocarbons from the air-hydrocarbon mixture providing hydrocarbon free air which may be vented to the atmosphere. Plural beds are required so that one bed may be regenerated while the other bed is on stream performing its adsorption function.
The basic concept of using activated carbon adsorbing beds in connection with recovering gasoline vapors was known commercially through the efforts of Rheem Superior Company which sold a Mark IV gasoline vapor recovery system in the early 1970's. One such Mark IV system was purchased by Texaco Company and has been operating at Texaco's Port Arthur gasoline loading terminal since at least 1975. The Calgon Corporation has also described, in its commercial material circulated in the 1972-74 period, systems utilizing plural carbon beds for collecting gasoline vapors. Calgon described such systems in connection with literature intended to promote sale of its activated carbon for use in such adsorber beds.
The prior art teaches the use of liquid seal vacuum pumps to desorb the hydrocarbons from such beds and further the use of absorbers to collect the concentrated hydrocarbon vapors removed from the beds by the liquid seal vacuum pump. The absorber is typically supplied with liquid gasoline as the absorbent liquid. The concept of liquid seal vacuum pumps and the associated absorber was also included in the gasoline vapor recovery apparatus sold by Rheem Superior as the Mark IV and installed in Port Arthur, Texas at the Texaco gasoline loading facility.
There have been variations and improvements over the Rheem Superior Mark IV unit which was on sale and in public use prior to any of these variations or improvements being patented or otherwise made known. The U.S. Pat. to McGill et al No. 4,066,423 relates to a hydrocarbon vapor recovery system which is substantially the same as the Rheem Superior Mark IV system, as far as the carbon beds, liquid seal vacuum pumps, and three-phase separator are concerned, but it differs radically in that it utilizes recovered hydrocarbon liquid as the absorbent liquid rather than fresh gasoline from storage as was done in the Mark IV unit. The U.S. Pat. to Dinsmore No. 4,276,058 discloses a hydrocarbon vapor recovery process and apparatus very similar to the Mark IV unit in which fresh gasoline used to cool the seal liquid for the liquid seal vacuum pump is circulated to the absorber to serve as absorbent liquid. The U.S. Pat. to Dinsmore et al No. 4,343,629 also teaches the use of cooling coils associated with the carbon beds to prevent temperature rises which might damage the carbon beds on some occasions.
In all of the foregoing prior art vapor recovery systems, including the Mark IV, the overhead gas from the absorber is conventionally recycled back to the on stream carbon bed to separate the remaining hydrocarbon vapors from the air with which they are mixed. It is important that this recycled overhead gas be maintained with as low a percentage as possible of hydrocarbons so as to minimize the loading in the on stream carbon bed.
In the design of the foregoing hydrocarbon recovery systems which are, for simplicity, called adsorption/absorption systems, it is, of course, desirable to minimize the size of the carbon beds, the vacuum pump and the absorber in order to construct the equipment at the lowest cost possible. The equipment is normally subjected to substantial loads during peak periods of utilization of the loading terminal. In between times, the terminal may be relatively inactive or completely shutdown. However, there is no capacity for storage of vapors so they must be processed continuously during the peak periods. The equipment typically operates on a cycle of approximately 15 minutes during which one carbon bed functions as an adsorber while the other carbon bed is being regenerated by the liquid seal vacuum pump. At the end of the 15 minute cycle, the regeneration of the one bed has been completed and, if the design capacity of the equipment is sufficient, the on stream bed will still have some limited capacity and will still be satisfactorily removing the hydrocarbons from the air-hydrocarbon mixture. When the beds are switched, the regenerated bed then becomes the on stream bed and the other is subjected to the regeneration or desorption process. In all of the commercially available adsorption/absorption hydrocarbon vapor recovery systems, the absorber is operated continuously as long as the vacuum pump is running.
In the initial Rheem Superior Mark IV hydrocarbon vapor recovery equipment, there had been an attempt to reduce the hydrocarbon content of the absorber overhead gas by terminating the flow of the gasoline hydrocarbon absorbent liquid for a very short time during the very end of the regeneration period, at which time purge air was being passed through the bed being regenerated. The theory behind this concept was that once the hydrocarbons were substantially removed from the bed being regenerated, there might be more air than hydrocarbons passing through the absorber causing the mixture to actually strip hydrocarbons from the absorbent liquid thereby producing an overhead gas having a higher percentage of hydrocarbons than the mixture delivered to the absorber. Therefore, Rheem Superior had proposed that the flow of gasoline absorbent liquid be terminated toward the end of the regeneration cycle so that such stripping would not occur and so that the overhead gas recycled to the on carbon bed would have a lower hydrocarbon content.
It is further noted that the U.S. Pat. to McGill et al No. 4,066,423 teaches the concept of maintaining a constant composition absorber overhead gas, as was true of most absorbers used for hydrocarbon vapor recovery. The present invention involves a process and apparatus in which the composition of the overhead gas is not maintained constant but is varied considerably in order to improve the efficiency of the system. By terminating the flow of the absorbent liquid at the initial portion of the regeneration cycle, it is possible to substantially decrease the hydrocarbons recycled to the on stream carbon bed.